J. Amer. Soc. Hort. Sci. 103(6): 846-849. 1978.
Pectin and Related Constituents in Avocado
Fruit during Ontogeny1
Irving L. Eaks and Walton B. Sinclair2
Department of Biochemistry, University of California, Riverside, CA 92521
ADDITIONAL INDEX WORDS.
Persea americana Mill., anhydrouronic acid, alcohol
insoluble solids, alcohol-soluble solids, oil
ABSTRACT.
Pectic substances in 4 avocado cultivars were determined as
anhydrouronic acid (AUA) during ontogeny and related to fruit maturity, alcoholinsoluble solids (AIS), alcohol-soluble solids (ASS), ASS minus oil, total oil, fresh
weight and dry weight. The concentration of pectic substances in avocado pulp
varied among different cultivars and increases during growth and maturation.
AUA varied between 0.7 to 1.5% on a fresh weight basis. However, values on a
dry weight basis are relatively constant at about 5.0% and independent of the
state of maturity or cultivar. AIS, ASS, alcohol-soluble acid and oil increase as the
fruit mature, ASS minus oil and water content decreased during the growth and
maturation periods. Change in oil content during ontogeny was the only
constituent of those examined which was related to maturity.
Pectic substances are usually associated with the tissue through which they are
dispersed as bonding material. Non-uronide constituents are closely associated with the
polygalacturonides. Highly purified pectic and pectinic acids frequently are found to
contain non-uronide constituents such as L-arabinose, D-galactose, L-rhamnose and
other neutral sugars. It is less likely to be dynamically related to oil or to other individual
systems of substrate which occur in macro-concentrations throughout the organism. On
the other hand, equilibrium between pectic substances and plant tissue constituting the
whole of the organism is necessary during the growth process and may serve as an
index to maturity.
Cellulose, protein, water, fatty ester (oil), simple carbohydrate and micro-concentration
of organic acids make up the greater portion of avocado fruit (9). Changes in sugars (1),
oil, water, fiber, protein, and carbohydrate (7) and pectin methylesterase and
polygalacturonase (10) during growth and development have been reported. Changes
in pectic substances during ripening have been determined (2, 5) but those in pectic
substances during growth and maturation have not been investigated. However, it is
expected that pectic metabolism in avocado will display a correlation positive to fibrous
tissue and to total dry matter and negative to individual systems of substrate during
1
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2
Professor of Biochemistry (Emeritus).
development. Knowledge of its position in the scheme of avocado growth and
maturation appears both necessary and useful in view of the ulterior role played by the
pectic substances in pathological metabolism in fruits (6).
Levels of pectic substances in 4 avocado cultivars were determined as the
anhydrouronic acid (AUA) and related to fruit maturity, alcohol-insoluble solids (AIS),
alcohol-soluble solids (ASS), ASS minus the oil, total oil, fresh weight and dry weight.
Materials and Methods
Representative 'Fuerte', 'Hass', 'Clifton' and 'McArthur' avocado fruit were picked
periodically and pulp from each cultivar was shredded to facilitate efficient isolation of
the fractional components. Ten g samples of pulp were treated for 4 or more hours with
95% ethyl alcohol in Soxhlet extraction assemblies. Alcohol-soluble and sparingly
alcohol-soluble solids, oils and water were separated from AIS in this manner.
All samples of AIS were washed with 75% ethyl alcohol. Several samples of each
cultivar were set aside, dried over CaCl2 and weighed. Each of the remaining samples
were dispersed in 200 ml 0.5% disodium versenate. Each solution was brought to pH
11.50 with dilute NaOH and saponification of all ester groups existing within the AIS
was accomplished in 48 hr at 24°C. This supposition is justified by the results of
preliminary experiments where the pH of a specific solution was measured at regular
intervals for 9 days. Cumulative increases in molar acidity indicated complete
saponification in 48 hr. Measurements were made following saponification, to determine
the change in pH in each solution and a molar value for saponified esters in the AIS
was calculated. The AUA monomer of a pectic chain as the theoretical molecular
weight was employed to obtain the percentage methylated anhydrouronic acid in the
samples. Finally, each solution was acidified with acetic acid to a pH of about 5.2.
Pectic chains were broken into monomers of galacturonic acid with pectinase and the
visible spectrum of the carbazole reaction described by McComb and McCready (3)
and by McCready andMcComb (4) was employed to measure AUA.
Standard procedures were employed to separate ASS and sparingly ASS from oils,
water and alcohol within each of the alcoholic extracts. Water, alcohol and trace
amounts of volatile oils were evaporated away in each instance under reduced
pressures at 80°C. Oils, traces of chlorophyll and other minor ether-soluble solids were
taken up in petroleum ether, following determination of the combined gravimetric weight
of oils and ASS and sparingly ASS in each sample. Ether-soluble oils were determined
gravimetrically after insoluble solids were filtered off and the solvent was evaporated
away with steam.
Water contents were estimated by the difference between sample weight and the
combined weight of the corresponding AIS and ASS and sparingly ASS which were
considered to be dry weight fraction. Alcoholic extracts of each cultivar were diluted with
H20 and total available alcohol-soluble and sparingly alcohol-soluble acid was
determined by titration with standard NaOH to a phenolphthalein end point. Oil contents
of the pulp of representative samples of the avocados were also determined by the
method described by Sweet (8).
Results and Discussion
ALCOHOL-INSOLUBLE SOLIDS.
The
pulp of avocado which remains
insoluble
in
alcohol
after
extraction consists principally of
polysaccharides; i.e., cellulose,
hemicellulose, starch, and pectin.
The last will be presented under a
separate heading later. AIS
expressed on a fresh weight basis
(Table 1) fluctuates throughout the
season, except for 'Hass' which
increased substantially during
maturation.
However,
AIS
expressed as a percentage of dry
weight vs. dry weight (Fig. 1) were
quite consistent with a slight
decrease in 'Hass' which had
relatively high dry weight. AIS
does not appear useful as a
maturity index.
ALCOHOL-SOLUBLE
SOLIDS.
The
soluble and sparingly soluble
portion of avocado pulp after
alcohol extraction can be divided
into 3 fractions; (a) water and
negligible amounts of volatile oil, (b) ether-insoluble solids such as simple
carbohydrates, acids and molecular constituents defined by electrovalent bonds and (c)
ether-soluble solids such as oils and negligible amounts of organic molecules defined
by covalent bonds. ASS increased in avocado pulp during growth and development
(Table 1) primarily because of the increase in the oil during maturation (Table 1).
Therefore, the ASS minus oil on a fresh weight basis (Table 1) decreased as the
season advances and the decrease is more striking when expressed as % dry weight
vs. dry weight (Fig. 1). The water content of all cultivars was relatively constant during
growth. Water comprised about 80% or more of the weight of immature avocado pulp of
all cultivars although some dependency is expected relatively to regional relative
humidity and irrigation practices. This value did not change significantly in 'McArthur' or
'Clifton' at maturity, but decreased in 'Fuerte' and 'Hass' at maturity (Table 1). The
alcohol-soluble acid as determined excludes a portion of the amino acids and includes
functional groups that behave in an acidic manner when treated with a strong base,
however the results provide estimates of total acid. The data show that 300 to 400
moles of hydrogen ion are present in 10 g of 'Fuerte', 'Hass', or 'Clifton' pulp before and
at maturity and that the acidity of 'McArthur' pulp increased with maturity from 520 to
680 moles. Therefore, acid in avocado is minimal and does not play an important role
nor is it useful as an index to maturity.
PECTIN.
Pectic substances expressed as % AUA on a fresh weight basis in the 4
avocado cultivars during ontogeny are listed in Table 1. The concentration of AUA in the
fresh pulp varied somewhat among cultivars and increased as maturity approaches.
However, variation in concentration of a dry weight basis from month to month or
among cultivars was negligible during growth and maturation (Fig. 2). AUA expressed
as % AIS varied only slightly during the season or between cultivars with a tendency to
be slightly higher at mid-season compared with young or mature fruit (Fig. 3). There
was an increase during the season when AUA is expressed as % ASS minus oil, which
decreased during the season and differed among cultivars (Fig. 4). 'Fuerte' and 'Hass'
showed similar patterns, with 'Clifton' being higher when mature and 'McArthur' being
lower throughout the later part of the season. Pectic substances expressed as % oil
decreased rapidly during growth and development and changed only slightly during
maturation for 'Fuerte', 'Hass', and 'McArthur' with only slight variations among these
cultivars (Fig. 5). 'Clifton' showed only a decline and the values were less than for the
other cultivars. Esterified AUA decreased as the fruit matured (Table 1). The
concentration of AUA increased during the season but the variation between cultivars
limits its use as a maturity index. However, the increase in AUA and the decrease in
esterification during the season is in agreement with reported lack of detectable
polygalacturonase and decrease in pectin methylesterase during development (10).
Changes in the concentration of other constitutents, in themselves or in relation to
pectic substances, appear to have no tangible relationship other than the role they play
as a part of the entire fruit.
Literature Cited
1.
Bean, R. C. 1958. Changes in sugars during growth and storage of avocados. Calif.
Avocado Soc. Yearb. 58:90-93.
2.
Doleno, A. L., B. S. Luh, H. K. Pratt. 1966. Relation of pectic and fatty acid changes
to respiration rate during ripening of avocado fruits. J. Food Sci. 31:332-336.
3.
McGamb., E. A., and R. M. McCready. 1952. Coloroimetric determination of pectic
substances. Anal. Chem. 24:1630-1632.
4.
McCready, R. M., and E. A. McComb. 1952. Extraction and determination of total
pectic materials in fruits. Anal. Chem. 24:1986-1988.
5.
Rouse, A. H., and C. R. Barmore. 1974. Changes in pectic substances during
ripening of avocados. HortScience 9:36-37
6.
Sinclair, W. B., and V. A. Jolliffe. 1961. Chemical change in the juice vesicles of
granulated Valencia oranges. J. Food. Sci. 26:276-282.
7.
Stater, G. G., S. Shankman, J. S. Shepherd, and R. B. Alfin-Slater. 1975. Seasonal
variations in the composition of California avocados. J. Agr. Food Chem. 23:468474.
8.
Sweet, R. H. 1955. Official method for the determination of oil in avocados.
California, Dept. Agr. Bul. XLIV: 37-41.
9.
Watt, B. K., A. L. Merill, and M. L. Orr. 1959. A table of food values. In Food, The
Yearb. of Agr. 1959. pages 213-266.
10.
Zauberman, G. and M. Shiffmann-Nadel. 1972. Pectin methylesterase and
polygalacturonase in avocado fruit at various stages of development. Plant Physio!.
49:864-865.